Hi adydye , well I am a collector of many years , based in the Uk and I have to confess I have never seen any thing like these!
To your questions, 1. I confess I don’t know the cause. possibly some lint picked up on a roller during a print run. It does not look like damage to the stamps surface 2. Does it affect the value?, these particular Coronation stamps are common and have minimal value , in this case I would suggest your flaws would actually INCREASE the value as a curiosity, rather than detract . 3 it is unlikely to be a regular flaw so will not be listed in any catalogue.